Toxoplasma gondii is an opportunistic pathogen that causes serious disease in immunocompromised patients. Most human infections are asymptomatic but immunosuppression due to organ transplant, cancer chemotherapy, or infection with HIV can lead to re-activation of the infection. In addition, infection of the fetus during pregnancy causes congenital toxoplasmosis. Some strains of T. gondii also cause severe ocular disease in immunocompetent patients. Treatment for toxoplasmosis is challenged by lack of effective drugs to eradicate the chronic infection and as many as 50% of the treated patients do not respond to the therapy. Most of the drugs currently used are poorly distributed to the central nervous system and they trigger allergic reactions in a large number of patients. There is a compelling need for safe and effective treatments for toxoplasmosis. Toxoplasma replicates inside its host cell and masterfully manipulates the host cell to insure favorable conditions for its survival and replication. T. gondii infection results in differential regulation of a variety of host signaling and metabolic pathways. Many of these host changes are not completely understood but it is quite likely that modifications of host pathways are essential for parasite growth and survival. Isoprenoids are the most diverse and abundant compounds occurring in nature. Many types of isoprenoids (e.g. steroids, cholesterol, retinoids, carotenoids, ubiquinones, prenyl proteins) are essential components of the cellular machinery of all organisms due to their roles in a variety of biological processes. All isoprenoids derive from a common precursor, isopentenyl pyrophosphate, and its isomer, dimethylallyl pyrophosphate, which are synthesized in mammalian cells via the mevalonate pathway. The human mevalonate pathway is the pharmacological target of statins and bisphosphonates, drugs used clinically to treat hypercholesterolemia and bone disorders, respectively. We found that very low doses of lipophilic bisphosphonates and combinations of statins and bisphosphonates, protected mice against a lethal dose of Toxoplasma. The synergistic interaction in vivo combining drugs (some used in the clinics), protected mice against death at a combination index 10 times lower than the fractional inhibitory concentration obtained in vitro. Recent work using cancer models revealed that certain statins or bisphosphonates have potent adjuvant activity in mice and monkeys by inhibiting geranylgeranylation of small GTPases, including Rab5, in antigen presenting cells, resulting in arrested endosomal maturation, prolonged antigen retention, and enhanced T cell activation. In addition, inhibiting the mevalonate pathway induces both a Th1 and cytolytic T cell response. Our hypothesis is that the effectiveness of the combination of statins and bisphosphonates for the treatment of T. gondii infection involves the participation of the immune system and that this strategy could be used against infection of other intracellular pathogens. We will characterize the participation of the host immune response in the synergistic effect obtained when combining inhibitors of host and parasite pathways.